Thermosetting powder coating compositions are well known in the art and are widely used as coatings for electric appliances, bicycles, garden furniture, accessories for the automotive industry, general metal parts and the like. Thermosetting powders consist of a mixture of a primary resin and one or more crosslinkers, often called hardeners or curing agents. The general approach associated with powder coating technology is to formulate a coating from solid components, mix them, disperse pigments (and other insoluble components) in a matrix of the major binder components, and pulverize the formulation into a powder. In so far as possible, each particle contains all of the ingredients in the formulation. The powder is applied to the substrate, usually but not limited to a metal, and fused to a continuous film by baking.
Compositions which include organic polyhydroxy compounds and blocked to include internal or self-blocked polyisocyanates and which are solid at room temperature, are important binding agents for thermally cross-linkable powder coatings (see for example, U.S. Pat. Nos. 3,857,818 and 4,375,539). Common to these systems is the disadvantage that, during thermal cross-linking, the compounds used as blocking agents, excluding self/internal blocked agents, are split off and escape into the environment. Therefore, during cure and crosslinking special precautions must be taken to purify the waste air and/or to recover the blocking agent for reasons of ecology and work hygiene.
The elimination of emissions from the curing of powder coatings has been attempted with the use of blocking-agent-free, uretdione-group-containing polyurethane (PUR) powder coating hardeners. In these compositions cross-linking takes place with thermal cleaving of the uretdione groups. (See for example U.S. Pat. Nos. 5,621,064 and 4,413,079). Typically, films produced with these types of uretdione crosslinkers, however, do not have optimal film properties such as hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss.
Another problem with powder coating compositions is that they frequently have low glass transition temperatures (T.sub.g) and will agglomerate or sinter when stored at elevated temperatures for a prolonged duration of time. This phenomena causes an application problem when the powder coating composition taken from storage is agglomerated and requires remilling, which may or may not permit application of a powder coating having a suitable particle size.
It is an object of the invention to provide a powder coating composition which has a relatively high glass transition temperature and that will resist agglomeration during storage.
It is an object of the invention to provide a powder coating composition which will maximize film properties such as hardness, flexibility, solvent resistance, corrosion resistance, weatherability and gloss, yet also provide a coating composition with a relatively high glass transition temperature.
It is another object of the invention to provide a powdered coating composition which can be cured at temperatures as low as about 160.degree. C. without the use of an effective amount of urethane catalyst or at temperatures less than about 160.degree. C. with the use of an effective amount of urethane catalyst such as 1,5-diazabicyclo(4.3.0)non-5-ene, 1,8-diazabicyclo(5.4.0)undec-7-ene, dibutyltin dilaurate, butane stannoic acid, dibutyltin oxide, and others known in the art.
It is yet another object of the invention to provide a thermosetting powder coating composition that includes crosslinker that does not release a blocking agent into the environment upon curing.
It is another object of the invention to provide a powder coating composition with OT bend performance, and accelerated cure schedules at temperatures greater than about 160.degree. C. which are typical requirements for coil coating applications.
It is another object of the invention to provide a powder coating composition with a desirable melt viscosity.
Other objects, advantages, features and characteristics of the present invention will become more apparent upon consideration of the following description and the appended claims.